Scientists have demonstrated a feat that your physics teacher may have told you was impossible -- they have created a material with a temperature below absolute zero. And the world below absolute zero is an unusual place indeed.

I. It's a Very, Very Mad World

Atoms float upwards, ignoring gravity. In a phenomenon that theoretical physicists believe mimic "dark energy", the atoms even stabilize in conditions that would normally crush inwards. It's as if gravity itself is being overridden and energetic arrangements that would normally create instability, instead stabilize. In short, we've entered the Twilight Zone of particle physics.

The MIT professor in a new interview with Nature lauds the new work by Ulrich Schneider of Munich, Germany's Ludwig Maximilian University and colleagues. In the work Professor Schneider demonstrates the first-ever peer-reviewed instance of a negative absolute temperature material breaking the absolute zero behavior.

The work began with Professor Schneider creating a peculiar quantum gas, using lasers and magnets. Composed of potassium atoms the gas was arranged into a lattice structure. A radical adjustment in the magnetic fields switches the atoms from the lowest energy state possible to the highest energy state possible.

Normally the stabilizing repulsion of the original configuration would be replaced with an immense attraction, causing the system to collapse and implode. But thanks to the trapping lasers, the lattice instead remains stable in the new super-energized state. Comments Professor Schneider, "This suddenly shifts the atoms from their most stable, lowest-energy state to the highest possible energy state, before they can react. It’s like walking through a valley, then instantly finding yourself on the mountain peak."

The result is a gas that by the formal definition of the Kelvin scale is a few billionths of a degree Kelvin below absolute zero (0 K).

II. Negative Absolute Temperatures? That's Really Cold, Right?

But don't be confused. The below-absolute-zero system is not cold. It is in fact very, very hot -- hotter than any positive Kelvin system. In cooler positive temperature systems, the numbers of particles in low-energy states outnumber those in high-energy states, giving rise to the formal quantum mechanics definition of temperature. Typically entropy pushes atoms to occupy lower energy states, on average.

But in certain specialized quantum mechanical systems, the entropy actually decreases as the system energy (and "heat") increase, giving rise to a negative quantum temperature.

In other words, to understand this wild breakthrough, you must abandon your traditional notions of negative being cold and positive being hot and think in quantum terms. This isn't your high school physics teacher's negative temperature. It's a bizarre exercise in inverted entropy.

Could such a state be possible for the faster-than-expected expansion of the universe (a phenomena cosmologist attribute to so-called "dark energy", a poorly understood mechanism)? Professor Schneider argues the idea is worth exploring. He comments, "It’s interesting that this weird feature pops up in the Universe and also in the lab. This may be something that cosmologists should look at more closely."

The work by Prof. Schneider and his colleagues was published in the highly prestigious peer-reviewed journal Science.

And if that makes your brain hurt, take a break and read the classic college urban legend of a physic professor's exam question of whether hell is exothermic or endothermic and his student's epic response.

After the quantum mechanical and statistical processes underlying thermodynamics were discovered, temperature was redefined as a factor relating the increase of energy of an isolated system to an increase in entropy (randomness). This definition allows negative temperatures.

For an ideal gas, added heat increases the average speed of the molecules, spreading the energy over a wider range of speeds. The wider range of speeds corresponds to an increase in entropy (or randomness). The temperature is positive because added heat increases entropy.

For very cold gases, quantum mechanical effects limit the speeds (or other energy-related quantities) to discrete values. For some gases, a small amount of added energy is not enough to jump the speed beyond a certain limit, so that added energy tends to produce speed values at or below the limit. This actually decreases the entropy because high speeds occur more frequently than low speeds (maximum entropy occurs when all speeds are equally likely). When an increase in energy reduces entropy, the modern definition of temperature is negative.

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